A SPIDAR system with seven Noggin 1000 systems deployed on a wheeled cart.

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PIDAR® systems network any number of Noggin and pulseEKKO PRO GPR systems together into a single data collection platform. While the possibilities are endless, the most common deployment is several systems with the same frequency spaced laterally to provide a swath of data in a single pass. This allows large areas to be surveyed in a fraction of the time compared to a single system.

In this case study, a client required imaging of the floor of an empty underground water reservoir undergoing refurbishment. The survey area was 50 by 100 meters with columns spaced roughly 5 m apart. The objectives were to identify the pattern of steel reinforcement (rebar) within the concrete floor and identify possible voids underneath the concrete. There was a concern that small water leaks in the reservoir could be causing voids to form and enlarge under the concrete. A custom-built SPIDAR® cart system was assembled with seven Noggin 1000’s spaced 0.1 meters apart. The cart was 1 meter wide with an integrated odometer wheel. The offsets between systems are entered into the SPIDAR® data acquisition software so that the relative position of each system is known for the entire survey.

With SPIDAR, the survey parameters for each system (time window [depth], step size, and stacks) can be different but in this case all systems had the same frequency, and used identical survey parameters:

Time Window = 22 ns (Depth = 1 m)
Step Size = 0.01 m
Stacks = 4

During data acquisition, each swath was aligned parallel to the last swath using a laser. A total of 48 line kilometers of data with 4.8 million traces and 1.5 billion data points was collected in 12 hours.

The data were processed and displayed as cross-sections and depth slices using the EKKO_Mapper software. The high resolution data show several features of interest:

• The concrete is reinforced with a double layer of rebar; the bars are on 12 inch centers and the two layers are separated 2-3 inches vertically (cross-section)
• The bottom layer is laterally offset from the top layer by 6 inches, with a rebar every 6 inches across the area (cross-section)
• A repaired patch near the upper right corner has no rebar in it
• No data were collected over the main pit drain, resulting in a blank blue area
• Drainage and outflow pipes are visible below the concrete in the cross-section and in a deeper depth slice
• The data showed no evidence of voids under the concrete